1. Field of the Invention
The invention relates to xerographic registration apparatus in general and in particular relates to registration method and apparatus for use in conjunction with a variable or multiple pitch copying machine.
2. Prior Art
In xerographic copying, a first step in the generation of a copy is the creation of a latent electrostatic image on a photoconductive material corresponding to light images of a document original. The latent image is then developed with toner material to render the latent image visible. This visible image is next transferred to a copy sheet at a transfer station and fixed to the copy sheet at a fusing station. It is of obvious importance that the visible toner image is in registration with the copy sheet at the transfer station so that the entire developed image is transferred to the copy sheet. It is also of equal importance that the image speed on the moving photoreceptor match the speed of the moving copy sheet to avoid a blurring of the imaging during transfer.
As the art of xerography has matured, different copier architectures have evolved. Certain high speed commercial xerographic copiers include belt or drum type photoconductors having image developing surface areas capable of holding multiple latent images about their periphery. The number of images which can be fit about the photoconductor depends upon the dimensions of both the photoconductor and the images supported thereon. The amount of space each image occupies including inter-image gaps is known as the copier pitch.
In many commercial copiers, the spacing or pitch occupied by images about the photoconductor is fixed. Since the typical document is imaged with its width dimension along the length of the photoconductor, so long as all documents have substantially the same width the pitch or spacing is constant. For a fixed pitch system the task of registering the copy sheet with the developed powder image is simplified. The photoconductor is driven at a constant rate so that the developed images approach the transfer station at a constant rate. If the copy sheets are driven to the transfer station at the same rate and the spacing between individual copy sheets is chosen to be equal to the photoconductor pitch once an initial synchronization between sheet and image is achieved only minor changes in the copy sheet drive speed are needed to maintain registration.
So-called multiple or variable pitch copier systems are also known. These systems copy document originals of differing widths so that the image spacing about the photoconductor periphery changes with document size. A photoconductor large enough to accomodate five images for one size document might only accomodate four document widths for a wider document. If the copier pitch changes, the timing of the copy sheet arrival at the transfer station must also change if a proper image transfer is to occur.
The variable pitch of a copier also affects the way the document is imaged onto the photoreceptor. In automatic high speed copying machines, document originals are fed automatically to a station for imaging on the photoreceptor. In certain instances it is important that the image of each sheet appears at a particular position about the multiple pitch photoreceptor.
The system disclosed in U.S. Pat. No. 3,888,579 to Rodek et al maintains document feed registration with respect to the photoreceptor by controllably accelerating or decelerating the document sheet by an appropriate amount, depending upon whether the sheet is lagging or leading its appropiate pitch frame location on the photoreceptor. The system employs a photodetector which identifies the passage of the leading edge of a document sheet at a registration point in the sheet path of travel. A comparator circuit utilizes this information to determine whether the document sheet is properly registered. If a misregistration is sensed, a correction is instituted through control of a drive stepping motor which either speeds up or slows down a drive roll by an amount required to place the documents in appropriate relation to the pitch frame on the photoreceptor.
While the '579 patent is limited in its disclosure to a mechanism for registering an original document to be copied, similar control techniques have application in copy sheet registration.
Applicability of document feed registration techniques for both original and copy sheet feeders has been recognized and in particular U.S. Pat. No. 4,170,791 to Daughton et al recognizes at column 10 that copy sheets can be either speeded up or slowed down to ensure that the sheet moves into contact with the photoreceptor drum at an appropriate speed and location.
The Rodek et al system which employs the stepping motor to either speed up or slow down the document feed apparatus has no feedback checking mechanism to insure that the steps taken to achieve registration are actually functioning properly. Wear in the system components and time delays in registration signal transmission can introduce sources of misregistration.
Proposals have been made to register documents using a servo drive system in conjunction with a feedback control technique whereby speed registration between a document and an image is continuously updated by known phase lock loop motor control techniques. The phase lock loop speed control proposals work well in a fixed pitch system, but cannot provide the speed and position registration needed in a variable pitch copier.
From the above it should be appreciated that while document feed registrations are known, and more particularly document feed registrations for use in conjunction with multiple or variable pitch copiers are known, prior art systems for achieving registration for such copiers have experienced difficulties in achieving accurate document feed registration. Prior art registration techniques have either been inaccurate or became inaccurate with use of the copier. Regardless of the cause, such misregistration is undesirable especially if good quality copies are to be obtained.